In the past, railroad switch machines were normally controlled by electromagnetic polarized relays. Generally, a typical interlocking installation contains four polarized relays with the windings connected in parallel to a pair of line wires which supply the required control voltages. In a crossover switch layout, two of the electromagnetic relays are utilized to control a first one of two electric switch machines while two other electromagnetic relays are employed to control the second of the two electric switch machines. In practice, a first polarity of d.c. voltage is applied to the pair of line wires to cause both of the switch machines to move to their normal positions while a second or opposite polarity of d.c. voltage causes both of the switch machines to move to their reverse positions. Thus, when the switch machines are in their normal positions, the trains are permitted to go straight through on both tracks; and conversely, when the switch machines are in their reverse positions, the trains are permitted to cross over from one track to the other track. It will be appreciated that it is only necessary to supply the d.c. voltage of any given polarity to the respective relay during the time that the switch machines are moving the points from one position to the other position. Once the switch points are shifted and locked, the polarized d.c. voltage may be removed from the line wires. Previously, the control circuits of the switch machines were actuated by fail-safe relay logic. With the present state of the art, it has become advantageous and desirable to construct interlockings having polarized output relays which are controlled by electronic microprocessors. However, since microprocessors are not per se fail-safe devices, it is mandatory to develop a manner that monitoring signals are fed back to the microprocessor. Thus, these feedback signals permit a check that the desired action has indeed been initiated and completed. The switch machines are designed to cause the closure of an electrical contact when the switch points are driven to their extreme positions and are locked in place. This contact closure will be used by the microprocessor to check that the desired action has taken place as commanded by the driving circuit.